Films made out of polypropylene random copolymers are very common within the polymer industry and are used in applications that require superior optical properties such as gloss, transparency, surface smoothness, planarity and good tear resistance.
Films made via blown film or cast film extrusion, are widely used for numerous applications, e.g. in food and textiles packaging, flower wrapping, as photo album page protectors, as coating substrates in extrusion coating processes or laminated to other materials in the formation of more complex films.
Such films are known for their good transparency, clarity or gloss and easy processability. They can be produced on high-speed extrusion lines, even with low thicknesses.
While the mechanical, chemical and processing properties make polypropylene the material of choice in a wide range of applications, its chemical inertness and low surface energy pose problems in applications that require printing, coating, bonding or adhesion with other substrates. For these applications it is therefore necessary to subject the polypropylene to a surface-modifying treatment that increases the surface energy.
The surface energy of a polymer is important for wetting and/or contacting a final article with inks, glue or coatings. The higher the surface energy the better the wetting will be.
Ideally the surface energy of the polymer is higher than the surface tension of e.g. the ink used for printing to get a good bonding between the ink, coating or glue with the respective surface.
In printing applications the surface energy needs to be increased from around 30 dyne/cm of untreated polymer articles to around 38 dyne/cm to be printable with solvent-based inks and to around 45 dyne/cm for being printable with water-based inks.
Increasing the surface energy of polypropylene is especially difficult as the energy required to increase the surface energy is much higher than for other polymers.
Suitable methods to increase the surface energy of polypropylene include chemical treatment, flame-treatment, and Corona-treatment. In flame-treatment the polypropylene surface is treated with an oxidizing gas flame. In Corona-treatment the polypropylene surface is treated with air that has been electrically ionized. All methods have in common that oxidized centers are generated, which facilitate the adhesion of inks etc.
Over time the obtained dyne-level will decrease. Within one week from treating the polypropylene a decay of 3 dyne/cm may occur. Subsequently the treated polypropylene may requires a “refresher” treatment before further transformation. This requires an additional handling step and additional effort for the packaging industry.
Hence it is a permanent need of the packaging industry, that the surface energy after surface treatment remains on an elevated level as long as possible, to provide more flexibility within the production and printing processes to achieve satisfying printing quality.